From reading their Wikipedia quickly sounds like BIS has a similar function to say the IMF when it comes to financial system stability. I do agree these sorts of organizations exert huge amounts of influence, especially for smaller countries that are dependent on loans and outside financing, but I'm not sure I agree they are more powerful than a nation itself. A nation can (theoretically) decide to opt out from these systems and operate independently, or can play different parties funded by nations (because in the end they all are working for someone's agenda) off of one another as many countries did during the cold war between the U.S. and Soviet Union. But if a nation reneges on its debt, the BIS, IMF, etc. isn't going to invade your country--one of it's creditor nations might, but not them.